Fig. 4.

Synaptically induced LTD occludes LTD produced by repetitive depolarization. A, Synaptic LTD was maximally induced by pairing 1 Hz stimulation with depolarization to −40 mV three times until no further LTD was elicited (openarrowheads). At this point, synaptic stimulation was stopped, and the neuron was repetitively stepped 20 times to +10 mV for 3 sec, once every 5 sec (arrow). No further LTD was apparent after repetitive depolarizations. Inset, The averages of five EPSCs taken at the times indicated on the graph are shown. Calibration: 200 pA, 10 msec. B, After maximally inducing LTD by pairing 1 Hz stimulation with depolarization to −40 mV two to four times until no further LTD was elicited, neurons were repetitively depolarized as described above (n = 7). LTD induced after repetitive depolarizations is 96 ± 8% of control values (n = 6). All experiments in this figure were performed in ACSF containing 5 mmCa²⁺.